Surface activities arms drills moles and mobility

While much can be achieved by purely passive observations and measurements of a planetary lander's immediate environment, some key science requires the landed system to interact with the surface mechanically. This may involve the acquisition of samples of material, either to be returned to Earth or delivered to instrumentation internal to the lander. Other instruments, while external, require intimate contact with target rocks - these include alpha-X-ray, X-ray fluorescence or Mossbauer spectrometers, and microscopes. Other interactions may include mechanical-properties investigations using a penetrometer, or current measurements of wheel-drive motors.

Thus a variety of mechanisms have been operated on planetary surfaces, including deployment devices and sampling arms of various types, together with drills, abrasion tools and instrumentation. Soviet/Russian landers have tended to feature simple but robust actuators, usually simple hinged arms, and often actuated by pyro or spring. These include the penetrometers on the Luna and Venera missions. Lunokhods 1 and 2 carried a cone-vane shear penet-rometer that was lowered into the lunar regolith and rotated by a motor, to measure bearing strength and shear strength. The rovers made 500 and 740 such measurements, respectively, during their traverses across the lunar surface.

A more sophisticated arm was flown on the Surveyor 3, 4 and 7 lunar landers (Figure 12.1). The Surveyor soil mechanics surface sampler (SMSS) was a tubular aluminium pantograph, five segments long, with a total reach of 1.5 m. As its name suggests, it was primarily a soil-mechanics experiment (indeed, in many ways the whole mission was primarily a soil-mechanics experiment). The strength properties of the soil, deduced also from landing dynamics, were inferred by measuring the motor current required to dig a trench in the ground. On Surveyor 7 the SMSS was mounted differently to enable it to pick up and reposition the alpha-scattering experiment on the lunar surface.

Figure 12.1. Space envelope of operation of the Surveyor 3 soil mechanics surface sampler (SMSS).

Figure 12.1. Space envelope of operation of the Surveyor 3 soil mechanics surface sampler (SMSS).

As an aside, lunar regolith is in fact a particularly nasty material to work with, having a wide range of particle sizes, and with grains being very angular (see e.g. Heiken et al., 1991). It is thus able to penetrate many mechanisms, and can be highly abrasive once it does so. (It is believed that difficulties in moving Surveyor 3's camera mirror may have been due to dust ingestion - its thrusters apparently kicked up clouds of dust at landing.)

The Viking lander surface-sampler arm was successful at delivering samples to the biological analysis instruments. It was, however, rather fragile. It used a shoulder joint to point an extensible boom (a coiled prestressed tubular tape, much like those used for booms and antennas on spacecraft). The multi-purpose scoop on the end of the arm is shown in Figure 12.2. Although it was used for some trenching and 'bulldozing' experiments (Moore etal., 1977), there were fears that it would be damaged in such operations. Viking Lander 1's robot arm was initially stuck until a pin was unjammed by repeated extensions (Spitzer, 1976).

The robot arm on Mars Polar Lander (Bonitz et al., 2001) and Phoenix, has (as for Viking) the principal function of delivering soil to experiments on the lander deck. The 4-degree-of-freedom 'backhoe' design has a 2.2 m reach. The 5 kg arm is capable of exerting considerable force on the ground (some 80 N, enough, in principle, to drag the lander!) in order to cut a trench in a possibly ice-rich soil. The sampling scoop is fitted with 'ripper tines' to tear through such potentially hard material. The arm also carries a camera for close examination of the soil.

An additional function on MPL was to emplace a temperature-sensing spike (mounted on the 'wrist') into the ground, and to vary the height of an air-temperature sensor to measure the boundary layer temperature profile. The Phoenix arm carries instead a thermal and electrical conductivity probe.

The Sojourner rover performed some soil mechanics experiments with its wheels (rotation and motor currents being recorded, as well as the resultant trenches being imaged by the rover cameras and/or the lander camera) (Moore et al.,

180° head rotation

Primary sieve (2mm

Hinged lid

180° head rotation

Primary sieve (2mm

Hinged lid

\AJV

Magnetic array External temperature sensor 'Squeegee' type brush

Figure 12.2. Viking lander scoop.

\AJV

Magnetic array External temperature sensor 'Squeegee' type brush

Figure 12.2. Viking lander scoop.

1999). In addition, it had an alpha-proton-X-ray spectrometer (APXS), which required direct contact with the rocks; emplacing this instrument was the rover's main scientific function (it was very much an engineering experiment overall).

The Mars Exploration Rovers had a similar overall goal, although with much more capability and instrumentation - an alpha-X-ray spectrometer being supplemented by a Mossbauer spectrometer and a microscope camera. Furthermore, rather than only attack exposed, and therefore generally dust-covered surfaces with these instruments, MER carried a rock abrasion tool (RAT - Gorevan et al., 2003), which could brush dust off, and grind a large shallow hole to allow depth-profiling. The RAT and other instruments were emplaced by a small arm, the instrument deployment device (IDD, Tunstel et al., 2005).

The Beagle 2 lander carried a well-instrumented robotic arm. At the end, a PAW (position adjustable workbench) was equipped with a stereo camera (with illumination), microscope, X-ray fluorescence spectrometer, Mossbauer spectrometer, rock corer-grinder, wide-angle mirror, wind-sensor and sampling device (Sims et al., 2003).

The amount of energy required (which can be inferred from motor currents) to drill a given volume of material is a quantity that can be considered a measure of the strength of the material, and is thus a diagnostic of the rock type. For example, a rather weak rock strength of 10MPa corresponds to 107 Jm~3 - thus to drill a 1 cm diameter hole to a depth of 10 cm requires roughly 80 J of energy -while a harder rock may require 20 times more energy.

Sampling (coring) drills were flown on the Luna Ye-8-5 and Ye-8-5M sample-return missions to acquire lunar soil samples for return to Earth. The first successful mission was Luna 16, which returned 101 g of material (Grafov et al., 1971), while Luna 20 returned 30 g. The drill was a thin-walled tube carrying helical threads on its outside surface and a crown on sharp teeth at its cutting end; it was insulated and sealed prior to its 500 rpm operation, to permit the use of conventional lubricants. The Luna 16 and 20 drills reached 25-35 cm depth. A more advanced drill, used successfully on Luna 24, reached some 2 m into the ground and collected 170.1 g of material. The stratigraphy of the regolith column was preserved by stowing the acquired sample in a coiled plastic tube.

Drills for Venus have been flown with some success by the Soviet Union (Venera 11 to VeGa 2, e.g. Barmin and Shevchenko, 1983) but are at an earlier stage of development elsewhere. The challenge is to acquire a sample at the high ambient temperature and pressure and transfer it to the interior of the lander. A recent innovation is ultrasonic (vibratory, rather than rotary) drilling.

A novel sample acquisition technology was flown on the Beagle 2 PAW. The PLUTO 'mole' is a self-hammering percussive drill (Richter, 2001) that winds a spring to push a free hammer. The mole derives originally from Russian technology (e.g. Brodsky et al., 1995; Gromov et al., 1997).

Mobility is often an important, desirable and enabling aspect to a planetary mission, allowing scientific targets beyond the craft's immediate environment to be reached. Mobility may be required in atmospheric, surface and sub-surface environments. Aerial mobility (e.g. balloons) and ice-penetrating 'cryobots' were addressed in Chapter 6. Mobility across a surface takes us into the field of robotics (e.g. Ellery, 2000) and planetary rovers (e.g. Kemurdzhian et al., 1993; CNES, 1993), which is deserving of a whole book in its own right; the details are beyond the scope of this work.

At the simplest end of the spectrum of complexity are relatively 'dumb' instrument-deployment devices, whose function is to transport sensors from a lander across the planetary surface beyond the radius accessible from the lander itself (e.g. by robotic arm). Such devices are usually tethered to the lander to provide power and data connections, which limits mobility but does minimize the need for power and communications equipment and autonomous control on the rover. The first such device flown was the PROP-M tethered walking rover flown on the Mars 2, 3, 6 and 7 landers in 1971 and 1973. All four missions were lost before PROP-M was to operate, however. Deployed by an arm from the lander, PROP-M was to perform penetrometry and densitometry measurements on the Martian surface material. It had the capability to sense (by means of 'whiskers' at the front) the presence of an obstacle, step backwards and turn to move around it.

The first planetary rovers were the two Lunokhod vehicles deployed by Luna 17 and Luna 21 in 1970 and 1973, following a launch failure in February 1969. These were teleoperated from the Earth (the relatively short two-way light time between the Earth and the Moon making this possible) and between them travelled a combined total of more than 47 km across the lunar surface. They returned many images and performed measurements of the lunar soil and surface environment, as well as carrying laser retroreflectors.

Two types of wheeled vehicle were used by the Apollo landings: the hand-drawn MET (modular equipment transporter, carried on Apollo 14 only) and the LRV (lunar roving vehicle, carried on Apollo 15, 16 and 17, see Cowart, 1973). An extensive literature exists concerning wheel-ground interaction ('traffic-ability') for lunar and planetary rovers (e.g. Bekker, 1962; Carrier et al., 1991) and rover dynamics more generally (e.g. Avotin et al., 1979; Kemurdzhian, 1986).

The first successful Martian rover was of course Sojourner (Mishkin, 2004), followed by the much larger Mars Exploration Rovers, Spirit and Opportunity (Squyres, 2005). In 2009 an even larger Martian rover, the Mars Science Laboratory, is planned to be launched, while ESA is planning to launch its own Martian rover on the ExoMars mission, due no earlier than 2013. Further lunar robotic rovers from the US, Japan and China are also in the early stages of planning as part of lunar landing missions.

For sample-return missions, additional challenges are introduced. For example, the constraints of the delivery and stability of the lander/ascent assembly are such that the ascent stage may be less slender than is optimal in the case of Mars; the ascent stage may incur a higher aerodynamic loss than would otherwise be the case. For Venus, aerodynamic losses are so large that any contemplated sample return mission would first use a balloon to climb above the densest part of the atmosphere before using a rocket stage. Unless the mission duration is very short, storable (i.e. non-cryogenic) rocket propellants need to be used. On the other hand, it has been proposed to perform in situ propellant production on Mars - to derive oxygen from the CO2 atmosphere and thus only require the delivery of the chemical processor, a power source to drive it and a fuel.

To date, however, ascents from other planetary bodies have been relatively limited in sophistication. Until recently, they were confined to the Moon - the Luna 16, 20 and 24 sample return missions (and eight other, unsuccessful missions of the Ye-8-5 and Ye-8-5M craft), and the Apollo landers. In all cases the landers were squat vehicles and served as launch pads for the ascent stages. While the Apollo vehicles had sophisticated (astronaut) guidance, the Luna missions were confined to a longitude of 60° E where a vertical ascent assured a direct return to Earth without further course adjustment. Both the Lunas and Apollo used storable bipropellant engines (nitric acid/UDMH and dinitrogen tetroxide/ UDMH, respectively, UDMH being unsymmetrical dimethyl hydrazine).

Surveyor 6 took off briefly from the surface of the Moon, to land about 3 m away thus enabling it to image the footpad impressions from its original landing. This is a reminder that, in general, any system capable of soft landing may also have the ability to lift off again (subject to the ignition characteristics of its engines).

For Mars sample return, studies to date require an Apollo-like orbital rendezvous. This avoids the need to carry the interplanetary return propulsion down to the Martian surface, and to inject the Martian ascent stage back to Earth. Isolating the sample in a small capsule also has planetary protection advantages.

Ascents from small bodies are easy, possibly too easy (the Philae lander includes a hold-down thruster to prevent the lander drifting away in the low gravity as it is anchored onto the surface by harpoon). Indeed, jumping is a convenient, albeit risky, way to achieve surface mobility in low gravity (e.g. Kemurdzhian et al., 1988; Richter, 1998; Yoshimitsu et al., 2003; Scheeres, 2004). To date, the only example of an ascent has been the recent Hayabusa mission, which appears to have landed and taken off twice from Itokawa, if not perhaps with the full participation of ground controllers.

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